Glaucoma is a group of diseases of the eye which, worldwide, is among the leading causes of irreversible blindness. The major risk predictor for the diseases is increased intraocular pressure (IOP). Therefore, risk profiling for, and management of, glaucoma includes measurements of IOP via tonometry.
It is generally accepted that the normal range of IOP is 10 to 21 mmHg, and individuals with an IOP higher than this range will usually take IOP lowering medication to prevent or delay the progression of glaucoma. Also, IOP for the individual can vary throughout a 24 hour period depending on whether the individual is asleep or awake, the level of physical exertion or hydration, and the psychological state. Therefore, measuring IOP only during irregular daytime periods, such as during two or three clinic visits a year, does not provide sufficient information for proper management of the disease.
A common method of measuring IOP is applanation tonometry which measures IOP whilst flattening a constant area of the cornea using a solid surface applied to the cornea. Since contact is made with the cornea, an anaesthetic must be introduced onto the surface of the eye. Apart from the discomfort for the subject, clinical technicians are required to take the measurements. This also entails that measurements will be taken only during the daytime office hours, which does not capture the natural variation of IOP and its increase during sleep and does not provide sufficient information for proper management of glaucoma. Other methods of measuring IOP exist which involve non-contact tonometry, but these suffer from the same or other disadvantages.
Furthermore, all pressure measurement devices (contact and non-contact) used to measure IOP are affected to different extents by the stiffness of the cornea. Thus, for example, a cornea with high stiffness, caused e.g. by high thickness or rigid tissue, would lead to an overestimation of IOP, and could result in a false positive diagnosis of glaucoma. Corneal stiffness varies from individual to individual due to variations in factors including thickness, curvature, age and medical history. Therefore, measured values of IOP require correction or calibration to account for these natural variations in stiffness.
The IOP varies with systemic blood pressure, such that there is an oscillating variation in IOP, termed the “ocular pulse amplitude” (OPA), which is defined as the difference between the maximum and minimum values of the ocular pulse wave. The OPA is of interest in management of glaucoma, since it provides a measure of the pressure range to which the eye is subjected. Recent studies suggest significantly higher OPA values in the eyes of glaucoma patients than in the eyes of healthy subjects.
It is desirable to provide a device for measuring IOP which substantially avoids discomfort for the subject or the need for clinical technicians and the like to perform the measuring process.
It is desirable to provide a device for measuring IOP which allows multiple measurements to be taken over at least a 24 hour period. It is also desirable to provide a device for measuring IOP which allows substantially continuous measurements to be taken. It is further desirable to measure OPA and its variation profile over at least a 24 hour period. It is also further desirable to eliminate the effect of corneal stiffness on IOP measurements to improve the accuracy of IOP output and remove the need for calibrating the tonometry device for every patient.
It is known that the cornea changes in structure as the distance from the centre of the cornea increases. In a central region, up to a diameter of approximately 7 mm, the cornea includes collagen fibrils which predominately have a vertical or horizontal orientation. In an outer region beyond this, starting from a diameter of about 11 mm, the collagen fibrils have a predominantly circumferential orientation. It has been found that the transitional region, having a diameter from about 7 to about 11 mm, between the central region and the outer region is characterised as being an area of low stiffness. Therefore, deformations in the cornea due to changes in IOP will be most apparent at this location, and in the radial direction.
It is known (e.g. U.S. Pat. No. 5,179,953; U.S. Pat. No. 4,089,329) to provide a semi-rigid ring or contact lens which applanates the sclera of the eye and holds a pressure sensor such as a strain gauge in contact with the sclera to measure IOP. Due to the requirement to cause applanation of the naturally stiff sclera, these devices are bulky and uncomfortable to wear. Other techniques rely on the use of corneal contact lenses equipped with pressure sensors, but these either measure IOP at a single point and require patient activation (US 2007/0129623), or measure IOP with circumferential pressure sensors, the placement of which is based on the geometric form of the cornea without consideration of the cornea's microstructure and its particularly low stiffness at the transition region and in the radial direction (WO 03/001991).
It is desirable to provide a device which measures IOP and/or OPA at or near the transitional region of the cornea. It is desirable to provide a device for measuring IOP which avoids causing patient discomfort and avoids obstructing the vision of the subject.
In published patent application WO 2010/061207 there is described a device which seeks to meet some of these requirements and which discloses various embodiments of a contact lens having an in-built pressure sensor.
It has been found that further improvements can be made which increase the accuracy and consistency with which the device interacts with the cornea and assesses intraocular pressure.